There are many situations where it is extremely important to be able to generate signals whose frequencies are highly defined. For example, the frequencies of the signals generated by the oscillators incorporated in each Bell System TOUCH-TONE telephone must fall within the passbands of the several filters located in a central office if they are to perform the dialing function for which they are intended.
One can readily design appropriate oscillators for this purpose. However, the effective design of any circuit that is to be mass-produced and subjected to a wide range of environmental conditions must go beyond the specification of a nominal circuit. It must include considerations of performance deviations due to parameter variations stemming from manufacturing tolerances and environmental changes. When this is done, one may find that the particular circuit design fails to meet the necessary requirements (See "A Monte Carlo Tolerance Analysis of the Integrated, Single-Substrate, RC, TOUCH-TONE Oscillator" by P. Balaban, B. J. Karafin and Mrs. D. B. Snyder, published in the April 1971 issue of the Bell System Technical Journal, pages 1263-1291).
One possible way of improving oscillator performance is to tighten the tolerance on all the oscillator components. It will be readily recognized, however, that such a procedure will tend to increase the cost of the telephone and, hence, is to be avoided if possible. Secondly, even if more carefully designed, severe environmental changes (i.e., high or low temperatures) may nevertheless cause significant changes in the signal frequencies.
A second situation wherein frequencies must be highly defined is in the area of echo cancellation. An echo occurs in a communication system when an electrical signal encounters an impedance mismatch. As a result of this encounter, the incident signal is partially reflected back towards the signal source. Because of the transmission time involved, the reflected signal, or echo, is heard some time after the incident speech was transmitted. As distances increase, the echo takes longer to reach the talker, and becomes more and more annoying. Accordingly, various means have been proposed for eliminating echoes in long distance communication systems. One such means extracts a portion of the incident signal and combines it out of phase with the reflected signal so as to produce a cancellation of the latter. The difficulty with this arrangement, however, resides in the fact that in some systems the echo signal has experienced a slight shift in frequency and, hence, cannot be readily cancelled by the echo canceller.
As is evident in both of the above-described applications, problems arise because the frequencies of interest are not synchronized relative to each other, or relative to the frequency characteristics of some circuits. This would suggest stabilizing the frequencies of interest relative to some reference frequency, and there are many circuit arrangements disclosed in the prior art for doing this. The difficulty with such an approach resides in the fact that a reference signal, as such, is not presently available in the telephone system. However, there are standardized signalling tones which emanate from the central offices. The problem is that these signalling tones are only available for short periods of time.
Accordingly, it is a first object of the present invention to synchronize a frequency by means of a reference signal of short duration.
It is a more specific object of the invention to use available signalling tones for frequency synchronization within the existing telephone network.